The high thermo-mechanical stress to which the piston of more advanced engines are submitted led to the need of incorporating local reinforcements, in order to maintain the structural integrity of the aluminum pistons.
Particularly at the region defined by the combustion chamber edge, where the highest temperatures and thermo-mechanical stresses take place, in order to increase the local strength of the aluminum alloy of said pistons, it is common to incorporate porous ceramic inserts that are infiltrated by the aluminum base alloy. As a result from the manufacturing process used for obtaining said insert, usually the squeeze casting process for aluminum alloys, a composite of high structural strength is produced.
Due to the characteristics of the reinforcing material, the thermal expansion of the composite material is inferior to that of the base material of the piston (aluminum alloy).
In case the insert presents the usual annular shape defining the peripheral wall of the piston combustion chamber, a serious problem occurs at the interface between the low expansion material of the insert and the piston remaining material made of aluminum alloy of higher expansion. Due to the difference between the thermal expansion coefficients of both materials, there occur stresses, resulting from the difference between the expansion of both materials under the same temperature.
Since the insert is in the form of a ring, which presents a generally rectangular cross section, which is disposed around the piston combustion chamber and which is projected practically throughout the whole height of the latter, from the upper end face of said piston, the interface defined between the lower end face of the insert and the adjacent radial face of the piston material made of aluminum alloy is disposed at a region submitted to high temperatures, as said region is close to the bottom of the piston combustion chamber.
The high temperatures at the piston region defined along the radial extension of said lower interface of the insert tend to force the piston aluminum alloy to expand radially, at values different from those of the corresponding radial thermal expansion of the insert material, thus demostrating the occurrance of the stresses mentioned above, which reach values sufficient to cause cracks and even ruptures on the piston aluminum alloy.